Thermal transfer overprinting apparatus normally includes a thermal printing head having a linear or 2-dimensional array of thermal elements. In use the thermal printing elements are selectively energised in accordance with data representative of an image to be printed, e.g. the output data from a computer, or a scanning device. The thermal head is brought into contact with a ribbon or tape bearing a hot melt ink or wax, sandwiching the ribbon or tape between the thermal head and a substrate. The selective energising of the elements in the thermal head then initiates transfer of the hot melt ink from the ribbon to the substrate.
It is recognised by those versed in the art that the print quality provided by a thermal printing head is highly dependent on the pressure applied by the thermal head to the substrate being printed, via the ribbon.
Many different forms of apparatus have been proposed to control the pressure applied by the print head to the substrate. One common form of apparatus uses compressed air delivered via a pneumatic circuit, in combination with a solenoid operated device, to control the air pressure. This method has the drawback that it is difficult to vary the pressure setting to account for different qualities and/or different thicknesses of substrate to be printed.
Another form of apparatus is described in Japanese Patent Application No. 4128053 which teaches the use of resilient means in the form of a compressed spring to generate a pressure between head and substrate. Yet another example is described in British Patent Application No. 2 294 907 which teaches the use of a stepper motor, in conjunction with resilient means, to drive a printing head into contact with a substrate, for a predetermined number of steps, to achieve a desired pressure.
There are a number of drawbacks with the prior art described. Among these are that the pressure applied depends to an extent on the hardness of the substrate. This problem can generally be overcome by means of calibration, and control of the various means that are available to enter and store printing settings into a control computer. Another problem with prior art forms of apparatus is that the control mechanisms for applying pressure are one-sided in that they compensate for a substrate becoming thinner by stepping the head is down. It will be appreciated, however, that pressure will increase if the substrate becomes thicker. As a consequence, typical applications for printers of this type are restricted to substrates whose thickness is well controlled, and largely flat. Further, in use, the printing head must be withdrawn between prints, thereby resetting the pressure applied.
It is an object of the present invention to provide thermal printing apparatus which goes at least some way in minimising the above-mentioned problems; or which will at least provide a novel and useful choice.